Catalyst compositions based on well defined donor ligand containing metal complexes, referred to as post-metallocene complexes have been shown to give products having narrow molecular weight distribution and high molecular weights. However, these catalysts often have poor high temperature stability and suffer from poor catalytic efficiencies, especially at elevated polymerization temperatures. When employed to prepare propylene containing polymers, especially polypropylene, propylene/ethylene copolymers, and ethylene/propylene rubber (EP) modified polypropylene, the resulting polymer tacticity, molecular weight and catalyst efficiency are often deficient.
Examples of the foregoing post metallocene catalysts are disclosed in U.S. Pat. No. 6,827,976 and US2004/0010103, where Group 3-6 or Lanthanide metal complexes, preferably Group 4 metal complexes, of bridged divalent aromatic ligands containing a divalent Lewis base chelating group are disclosed for use in olefin polymerizations.
Higher solution reaction temperatures are particularly desired for propylene polymerizations in order to reduce energy consumption and improve operating efficiency. However, the use of higher reaction temperatures often results in poor conversions, lower polymer molecular weight, and reduced polymer tacticity. Accordingly, selection of catalyst compositions capable of formation of isotactic polypropylene at increased efficiency at elevated reaction temperatures is highly desired.
We have now discovered that certain metal complexes may be employed in a highly efficient solution polymerization process to prepare propylene polymers, including propylene homopolymers and copolymers of propylene with one or more comonomers. We have also discovered that a continuous process can be employed to prepare the foregoing propylene polymers in increased yield and efficiency. In particular, the present inventors have discovered an improved continuous solution polymerization process for preparing such polymers that is characterized by high polymerization efficiency and productivity.